Dynamic spinal screw-rod connectors

ABSTRACT

A transverse fixator assembly for spanning between a number of longitudinal members situated adjacent a patient&#39;s vertebrae and methods for fixation of the spine which allow variation of the distance between two or more vertebrae. The assembly includes a number of connectors configured to span the distance between and engage the longitudinal members. The connectors define a thru-hole for engaging a bone bolt which is engaged to a vertebra plus a number of spikes projecting from the connector. A locking mechanism is configured to prevent the bolt from rotating relative to the connector when the nut is being tightened. One or more of the connectors may be a dynamic connector which is slidably engaged to the longitudinal members to vary the distance between the vertebrae for compression or distraction.

This application is a continuation of Ser. No. 08/377,658 filed on Jan.25, 1995 and issued as U.S. Pat. No. 5,620,443 on Apr. 15, 1997.

FIELD OF THE INVENTION

The present invention broadly concerns devices for use in spinal implantsystems, particularly those using spinal rods contoured for connectionat various locations along the length of the spinal column. Morespecifically, the invention concerns an apparatus for spanning betweenspinal rods to support vertebral fixation elements of the implant systemwhich provide direct engagement to vertebrae of the spinal column. Theinvention is particularly useful with methods and devices for anteriorfixation of the spine.

BACKGROUND OF THE INVENTION

Spinal fractures often occur at the thoracolumbar junction. Most ofthese fractures are burst injuries which are particularly dangerousbecause retropulsed bone fragments can cause spinal cord or caudalequina injuries. Posterior fixation has long been the primary approachfor traumatic spinal injuries of this type.

The development of posterior internal fixation procedures for burstfractures was a substantial improvement over early approaches of bedrest and body casts. However, several disadvantages to these procedureswere discovered. For example, this approach fails to reduce kyphosis orallow complete clearing of the spinal canal. Other complications includepseudoarthroses, late rod disengagement and inadequate reduction. Also,some posterior instrumentations require the fusion to extend at leasttwo levels above and below the injury, particularly at the thoracolumbarjunction. The posterior approach is also limited in the viability foruse in burst fractures because in such fractures neural compressiongenerally occurs from the anterior direction. Therefore, it is better todecompress and fuse the spine anteriorly. These difficulties havemotivated attempts at anterior approaches. Various anterior andposterior spinal fixation devices and methods are discussed in Howard S.An, et al., (1992) Spinal Instrumentation, herein incorporated byreference.

There are several advantages to anterior internal fixation. An anteriorapproach allows complete clearance from the spinal canal of bonefragments and/or total resection of a tumor. It also permits fusion of aminimal number of motion segments. In spite of these advantages, the useof anterior approaches has been limited by the risk of complications andother disadvantages of current systems.

Several plate and screw systems have been designed for anteriorinstrumentation of the spinal column. The Syracuse I-Plate (Danek andSynthes) may use rigid or semi-rigid screws in combination with a plate.Distraction or compression of the bone graft is not possible with thissystem. The CASF Plate marketed by Acromed is designed to be used in asemi-rigid manner. This device, as well, does not permit compression ordistraction of the bone graft and in addition cannot be used in a rigidconstruct. The Stafix Plating System marketed by Daruma of Taipei,Taiwan, is an anterior thoracolumbar plate designed to address similarindications. This plate incorporates slots and holes well as permittingquadrilateral placement of screws. The Anterior Thoracolumbar PlatingSystem under development with Danek and Dr. Zdeblick is a slotted platedesigned to attach to the anterior lateral aspect of the vertebral body.The plate allows distraction and/or compression through the use of twoscrews and two bolts.

Several modular spinal instrumentation systems were developed foranterior instrumentation. The Kaneda device is a system which includes arod coupler distant from the point of attachment to the vertebralbodies. Rods are inserted through holes in the spinal screw heads whichare then attached to the superior and inferior vertebral bodies.Normally two screws are placed in each body, therefore two rods arerequired. These rods are threaded to allow compression and distractionand are connected to form a solid construct at the end of the procedure.The Texas Scottish Rite Hospital System is also a modular spinal systemwhich can be used anterioraly for the management for burst fractures ortumors. This device can be configured much in the same way as the Kanedadevice with two screws in the superior and inferior vertebral body, eachconnected by rods which are in turn connected together. The Dunn deviceis another anterior spinal fixation device for use in tumor orthoracolumbar burst fractures. This device, similar to Kaneda, involvesvertebral body staples, screws positioned in the vertebral body, and twothreaded rods connecting a superior and inferior vertebral body to forma rigid construct.

These systems have proved unsatisfactory. Many of these devices such asthe Syracuse I-plate and the Casp plate do not allow distraction orcompression of a bone graft in fusion cases. Such static systems cannotbe used to correct certain disorders such as kyphosis. The systems thatdo allow distraction and/or compression are often too complicated andinvolve the use of multiple screws and bolts. The prominent bone screwsand rods of some devices increase the danger of vascular injury.Hardware failures, such a screw pull-out, have led to complications,including pseudoarthrosis. Some systems are further limited because theycannot be used in a rigid construct.

It would therefore be desirable to have a low profile, streamlinedsystem with a minimum of separately implanted components to reduce theamount of time required to implant the system, the risk of vascularinjury and the problem of irritation to the surrounding soft tissue ofthe patient.

A need exists for devices for anterior fixation which reduce the risksof anterior fixation by providing a mechanism to prevent hardwarefailures, such as screw pull-out.

It is desirable to have a spinal fixation system that is readily adaptedto provide lateral coupling between spinal rods and multiple stages orsegments of the spinal column. Such a system should provide thissegmental interconnection without interfering with vertebral areasavailable for bone grafting to achieve permanent fixation orimmobilization of damaged vertebrae.

There is also a need for low profile, streamlined systems which allowvariation of the distances between vertebrae, i.e., compression anddistraction, without the need for complicated instrumentation and tools.

There is currently no system that addresses each of these features in asingle apparatus. The present invention addresses these needs andprovides other benefits not previously found in spinal fixation systemsof the prior art.

SUMMARY OF THE INVENTION

In accordance with the invention, an apparatus is provided for spanningbetween a pair of longitudinal members situated adjacent a patient'svertebrae along the sagittal plane. The assembly includes a number ofconnectors which are engageable to the longitudinal members via clampingsurfaces provided in a slot defined in the connector. Each of theconnectors defines a thru-hole for engaging a bone bolt which in turn isengaged to a vertebral body. A fastener clamps the bone bolt to theconnector. The assembly also includes a number of fixation spikesprojecting from the connector which are configured to engage thevertebrae.

In a specific embodiment of the invention, there is provided a lockingmechanism configured to prevent the bolt from rotating relative to theconnector and the vertebra when the nut is being tightened. The lockingmechanism may include radial splines on the lower surface of theconnector and also on a mating face on the bone bolt. In anotherembodiment, the spinal fixation system includes a dynamic, or movable,rod connector and a fixed rod connector which allows variation of thedistances between vertebrae for compression or distraction.

One object of the invention is to provide an apparatus for use inlaterally connecting longitudinal members implanted adjacent a patient'svertebral column.

Another object of this invention is to provide an apparatus whichprovides for convenient management of thoracolumbar burst fractures andtumors and which permits anterior load sharing as well as compressionand distraction.

One benefit of the apparatus of the present invention is that itcombines means for connecting the vertebral fixation elements to thespinal rods with means for laterally or transversely connecting thespinal rods together. An additional benefit is that the inventionprovides a more compact construct with a lower profile as compared toprior spinal rod constructs employing many individual components toconnect vertebrae and spinal rods.

Yet another benefit achieved by the invention resides in providingsegmental coupling or connection of the spinal rods, while permitting awide variation of orientations at the vertebral fixation elementsrelative to the spinal rods.

Another advantage of this invention is that it provides fixationassemblies that can be top loaded, or implanted over bolts after thebolts have been engaged in the vertebrae.

Other objects and further benefits of the present invention will becomeapparent to persons of ordinary skill in the art from the followingwritten description and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the spinal fixation system ofthe present invention including a pair of transverse connectors spanningbetween two spinal rods with a pair of vertebral fixation bolts andcorresponding nuts.

FIG. 2 is an end elevational view of a transverse connector according toone embodiment.

FIG. 3 is a side cross-sectional view of a transverse connector engagedto a vertebra.

FIG. 4 is a side elevational view of a bone bolt according to oneembodiment of the invention for use with the fixation system shown inFIG. 1.

FIG. 5 is a bottom elevational view of the lower surface of a transverseconnector shown in FIG. 1.

FIG. 6 is a top elevational view of the bone bolt according to oneembodiment as shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated devices, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The present invention is useful for anterior internal fixation of thespine which is indicated for thoracolumbar burst fractures withsignificant canal compromise, vertebral body tumors, lesions due toinfection, spondylolisthesis, degenerative discs, and post-laminectomyinstability.

This invention provides a top-loaded, low profile anterior fixationsystem which requires minimal instrumentation yet permits anterior loadsharing and compression or distraction. The unique constructs of thisinvention permit fixation and compression or distraction with only twobolts, two rods and two rod connectors.

A spinal fixation system 10 in accordance with a preferred embodiment ofthe present invention is depicted in FIG. 1. The system 10 includes atransverse connector 15 defining a thru-hole 16 and having a lower boneengagement surface 20 and an upper surface 23. The transverse connector15 engages a number of longitudinal members 11 by clamping surface 22provided in a slot 21 defined in the connector 15. Preferably thelongitudinal members 11 are spinal fixation rods. In one embodiment, themembers 11 are smooth shot peened rods.

Referring to FIGS. 1, 2, and 3, a number of fixation spikes 17 arefixedly attached to the lower surface 20 of the connector 15. The lowersurface 20 of the connector 15 in combination with an inner surface 19of the fixation spikes 17 are configured to fit snugly around eitherside of a vertebra. In another application, the fixation spikes 17 maybe slightly embedded into the vertebra. The end of each spike 17 ispreferably beveled on its outer surface 24 so that each fixation spike17 terminates in a wedge shape 18 which may aid in fixing and holdingthe connector 15 in place over a vertebra.

Bolts 30 are used to attach the system 10 to the vertebrae. It isunderstood that "bolt" refers to any of various bone fasteners,including a standard bone screw. The present invention is unique becauseit requires only one bolt per connector. Previous devices have requiredtwo. FIG. 4 shows one embodiment of a bolt 30 in detail. The bolt 30 hasa vertebra engaging portion 31 at a first end 33 and a post 32 at asecond end 35. The vertebra engaging portion 31 of the bolt 30 may beconfigured, for example, with cancellous threads for fixation in thespongy bone of the vertebral body. The bolt also includes an integralflange 36 for supporting and clamping the connector 15. In oneembodiment, the second end 35 is configured to receive a driving tool.The configuration may include an internal or external hex as is wellknown in the art.

The fixation system 10 can be top-loaded, i.e., implanted over a bolt 30after the bolt 30 is engaged to a vertebra. This is advantageous becauseit reduces the required size of the surgical opening and trauma to thepatient. Top-loading also provides a mechanical advantage duringimplantation of the system. After a bolt 30 is engaged to a vertebra byconventional means, the post 32 is insertable through the thru-hole 16of the connector 15. A fastener 40 is provided for each of the bolts 30.The fastener 40 engages to the post 32 of the bolt 30 to secure theconnector 15 to the bolt 30 and to clamp the longitudinal members 11within the slot 21 of the connector 15. Thus, the longitudinal members11 and the connector 15 are secured by a single bolt 30. Where thefastener 40 is a threaded nut, as in the preferred embodiment, the post32 of the bolt 30 may included machine threads to engage with the nut.The nut or other fastener 40 is then top-tightened with a tool such as asocket wrench.

The connector 15 may define recesses 41 surrounding each thru-hole 16defined in the connector 15. Each recess can be configured to accept afastener 40 in low profile so that the fastener 40 does not extend overthe upper surface 23 of the connector 15 when it is engaged to a posts32. The recesses 41 can be concave to accept an arcuate underside of thefastener 40.

The spinal fixation system 10 may also be provided with a lockingmechanism configured to prevent the bolts 30 from rotating relative tothe connector 15 and the vertebra when the fastener or nut 40 is beingtightened onto the bolt 30. The locking mechanism also prevents thebolts from pulling out over time. In one embodiment of the invention,the locking mechanism includes an annular ring 45 defined on the lowersurface 20 (FIG. 5) of the connector 15 and a mating face 50 affixed toeach bolt 30 at a location between the post 32 and the vertebraeengaging portion 31. The annular ring 45 on the lower surface 20 of theconnector 15 is concentrically disposed around the thru-hole 16 andincludes a number of radial splines 46. Referring to FIGS. 1 and 6, themating face 50 is concentrically disposed around and affixed to the bolt30 and includes a number of opposing radial splines 51 forinterdigitated engagement with the radial splines 46 on the lowersurface 20 of the connector 15. The annular ring 45 may alternately be awasher affixed to or a ring integrally formed on the lower surface 20 ofthe connector 15.

The clamping surface 22 provided by the slot 21 defined in the connector15 may include a number of scallops (FIG. 6). The scallops areconfigured to receive the longitudinal members 11 in a manner that iswell known in the art. For example, each scallop can be generally formedat a radius that is slightly smaller than the radius of the longitudinalmember 11 which is to be situated within the scallop. The scallopsprovide means for fixing the spinal rods so that the longitudinalmembers 11 and connector 15 do not shift relative to each other.However, it is understood that the slot 21 defined in the connector 15may be smooth and that other means may be provided to firmly fix thelongitudinal members 11.

For example, in one embodiment, the slot 21 is smooth and the engagementof the longitudinal members 11 with the connector 15 is secured by aclamping action. The tightening of a fastener 40 on the post 32 causes anarrowing of the slot 21 of the connector 15 which in turn causes theconnector 15 to securely clamp the longitudinal members 11.

Another aspect of this invention provides means to vary the distancebetween vertebral bodies. According to the invention, a dynamictransverse connector 55 (FIG. 1) is slidable along the two longitudinalmembers 11 for compression and distraction of the vertebral bodiesattached to the system 10. One or more of other connectors 15 may beengaged to the longitudinal members at a fixed location. Aftercompression or distraction is achieved, the dynamic connector 55 can befixed by tightening the bolt 30 to which the connector 55 is fastened.The nuts 40 which attach to the bolts 30 can then be top tightened witha tool such as a socket wrench.

The invention also provides methods for fixating the spine which includedrilling a first hole in a first vertebral body and drilling a secondhole in a second vertebral body. A bone bolt 30 is engaged to each ofthe first and second holes. The vertebrae are then supported with afixation system 10 which includes two longitudinal members 11, such asrods. A first connector 15 is attached to a first end of each of thelongitudinal members 11, and a dynamic rod connector 55 is slidablyengaged to the longitudinal members 11. One of the bone bolts 30 isengaged to the thru-hole 16 of the first connector 15. The dynamic rodconnector 55 is situated so that another bolt 30 engages a thru-hole 16in the dynamic rod connector 55. The dynamic rod connector 55 is thenslid along the longitudinal members 11 to vary the distance between thefirst and second vertebrae. The post 32 of each bone bolt 30 is thenengaged with a nut 40 to secure the fixation system 10 to the vertebrae.The dynamic rod connector 55 may be slid along the longitudinal members11 in the direction towards the fixed connector 15 to compress thevertebrae before engaging the bone bolts 30 with the nuts 40. Thedynamic connector 55 may also be slid in a direction away from the fixedconnector for distraction.

The spinal fixation system 10 is preferably formed of medical gradestainless steel or similar high strength material. Other materials arecontemplated, provided the material is strong enough to endure the highloads transmitted through the components, and yet are biocompatible.Specifically, the system could be manufactured in 6A14V titanium or316LVM stainless steel. The system can be provided in several differentsizes ranging from, but not limited to, 2.0 inches to 5.5 inches.

While the invention has been illustrated and described in detail and thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. A spinal fixation system comprising:at least twolongitudinal members; a one-piece transverse connector defining athru-hole and havinga lower bone engagement surface, an opposite uppersurface, and said connector defining a continuous slot intersecting saidthru-hole to receive said members and a number of internal clampingsurfaces surrounding said slot, said clamping surfaces configured toengage said members; a bolt havinga vertebra engaging portion on a firstend, a post on an opposite second end, said post insertable through saidthru-hole of said connector, an integral flange between said vertebraengaging portion and said post for supporting said connector; fixationmeans for provisionally holding said connector in engagement with afirst vertebra; a fastener engageable to the post of said bolt to securethe connector to the bolt and to clamp said connector between saidfastener and said flange of said bolt; and second means engageable to asecond vertebrae and fixable to said longitudinal members for varyingthe distance between the first vertebra and a second vertebra forcompression or distraction of the spine.
 2. The system of claim 1wherein the post of said bolt is threaded and the fastener is a threadednut.
 3. The system of claim 2 further comprising a locking mechanismconfigured to prevent the bolt from rotating relative to said connectorand the vertebra when the threaded nut is being threaded onto said post.4. The system of claim 3 wherein said locking mechanism includes:anannular ring defined on the lower surface of said connector andconcentrically disposed around the thru-hole, said annular ring defininga number of radial splines; and a mating face affixed to said flange andconcentrically disposed around said post of said bolt, said mating facedefining a number of opposing radial splines for interdigitatingengagement with the radial splines on said lower surface of saidconnector.
 5. The system of claim 4 wherein the annular ring isintegrally formed on the lower surface of said connector.
 6. The systemof claim 4 wherein the annular ring is a washer affixed to the lowersurface of said connector.
 7. The system of claim 1 wherein the secondend of said bolt is configured to receive a driving tool for driving thebolt into a vertebra.
 8. The system of claim 7 wherein the second endincludes an external hex configured to receive a driving tool.
 9. Thesystem of claim 7 wherein the second end includes an internal hexconfigured to receive a driving tool.
 10. The system of claim 1 whereinsaid thru-hole includes a recess defined in the upper surface of saidconnector and said fastener is sized to be received in a recess withoutextending above the upper surface.
 11. The system of claim 10 whereinsaid recess is concave and said fastener includes an arcuate underside,said recess configured to accept said fastener.
 12. The system of claim1 wherein the clamping surfaces are defined by a number of scallopsformed in said slot, said scallops shaped to receive and engage saidmembers therein, said scallops surrounding and clamping a portion ofsaid members after the nut has been tightened on the post of the bolt.13. The system of claim 1 wherein said fixation means includes a numberof spikes projecting from said lower surface of said connector, saidspikes configured to engage a vertebra.
 14. The system of claim 1wherein said second means includes a dynamic rod connector engageable toa second vertebra in the spine and having a lower surface and an uppersurface and defining a thru-hole, said dynamic rod connector slidablyengaged to each of said rods and moveable relative to said fixed rodconnector to vary the distance between the first and second vertebrae.15. A spinal fixation system comprising:two substantially parallel rods,each having a first end and a second end; a fixed rod connectorengageable to a first vertebra in the spine and having a lower surfaceand an upper surface and defining a thru-hole, said fixed rod connectorimmovably fixed to the first end of both of said rods; a one-piecedynamic rod connector engageable to a second vertebra in the spine andhaving a lower surface and an upper surface and defining a thru-hole,said dynamic rod connector slidably engaged to each of said rods andmoveable relative to said fixed rod connector to vary the distancebetween the first and second vertebrae, said dynamic rod connectorfixable to said rods to fix the distance between the vertebrae; one boltfor each connector, each said bolt having a vertebra engaging portion ona first end and a threaded post on an opposite second end, said postinsertable through said thru-holes of said connectors; each said boltincluding an integral flange between said vertebra engaging portion andsaid post for supporting a corresponding connector; and a threaded nutfor each of said bolts, one nut engageable to one bolt to secure saidfixed rod connector to said one bolt and the other nut enaageable to theother bolt to compress said dynamic rod connector about said rods and tosecure said dynamic rod connector to said other bolt.
 16. The system ofclaim 15 wherein the second end of said bolt is configured to receive adriving tool for driving the bolt into a vertebra.
 17. The system ofclaims 15 wherein the second end includes an external hex configured toreceive a driving tool.
 18. The system of claim 15 wherein the secondend includes an internal hex configured to receive a driving tool. 19.The system of claim 15 wherein each said thru-hole includes recessesdefined in the upper surface of said connector and said nut is sized tobe received in a recess without extending above the upper surface. 20.The system of claim 19 wherein said recess is concave and said nutincludes an arcuate underside, said recess configured to accept saidnut.
 21. The system of claim 15 wherein said dynamic rod connectordefines a continuous slot to receive said rods and said slot includesinternal clamping surfaces for engaging said rods, the clamping surfacesdefined by at least two of scallops formed in said slot, said scallopsshaped to receive and engage said rods therein, said scallopssurrounding and clamping a portion of said rods after the nut has beentightened on the post of the bolt.